Anaerobic infectionsAnaerobic infectionsPART 3: Infection with Gram-negative obligate

anaerobes (Bacteroides spp. and other abscess-forming bacteria)

Prof. Cary Engleberg, M.D.Division of Infectious Diseases,Department of Internal Medicine

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What are these lectures about?What are these lectures about?• Clostridium spp.

– Gas gangrene/myonecrosis– Tetanus– Botulism– Antibiotic-associated colitis

• Bacteroides spp.– Abscesses

• Other obligate anaerobes

C. perfringens, C. septicum,C. histolyticum, C. novyi, etc.

C. tetani

C. botulinum

C. difficile

B. fragilis, B. distasonis,B. thetaiotamicron

Fusobacterium, Prevotella,Porphyromonas spp.

Bacterial species in the colonBacterial species in the colonpresent in >90% of fecal specimenspresent in >90% of fecal specimens

Data Source: Mandell et al. Principles & Practice of infectious Diseases

Bacterial CategoryLog organisms/gm

(dry weight)Range of log

organisms/gm (dry weight)

Bacteroides 11.3 9.3 – 13.8Eubacterium 10.8 5.1 – 13.6Anaerobic cocci 10.7 4.0 – 13.6Clostridium 9.9 4.0 – 13.2Streptococcus 8.9 3.9 – 12.8Gram-negative facultative

8.7 4.0 – 12.5

Other facultative organisms

6.8 1.0 – 12.5

Composition of FecesComposition of Feces

Undigested debrisUndigested debrisBacteriaBacteria

Gram− obligate anaerobesGram− obligate anaerobesGram+ Gram+

obligate obligate anaerobesanaerobes

All facultative bacteria(Gram + and Gram -)

Gram Stain of FecesGram Stain of Feces

Gram-negative obligate anaerobes

Gram-negative facultative bacteria

Case: appendicitisCase: appendicitis• An 18-year-old college freshman comes to the hospital with

diffuse abdominal pain, diarrhea, and nausea without vomiting. Pain is localized to the right side of the abdomen.

• P.E.: tenderness with rebound tenderness over the right lower quadrant.

• She is treated with a 1st generation cephalosporin• She is taken to surgery where a perforated appendix is

removed. The surrounding peritoneum is irrigated.• Cultures of the peritoneum grow a mixture of bacteria,

typical of those found in stool.

Case (continued)Case (continued)• On post-op day #2, her temp spikes to 38.6°C. • Blood cultures obtained preoperatively grow E. coli.• She completes a 7-day course of cefazolin and improves.

Since she has no further symptoms and follow-up blood cultures are negative, the antibiotic is stopped.

• 36 hours later, her temperature is 38.8°C and she feels diffuse pain over the site of the appendectomy.

• A CT scan of her abdomen reveals a retroperitoneal abscess.

CT scan: Ruptured AppendixCT scan: Ruptured Appendix

Yu J et al. Am J Roentgenol. 2005;184(4):1136-1142.

Case (continued)Case (continued)

• The abscess is drained, and cultures of pus from the drainage grow Bacteroides fragilis.

• She is treated with ampicillin-sulbactam for 14 more days. Her drain is pulled after 7 days, and she has an uneventful recovery.

Gram stain of drainageGram stain of drainage

Source undetermined

B. fragilis B. fragilis in pure culturein pure culture

CDC/Public Health Image Library, Dr. V.R. Dowell Jr., #3084

Questions to considerQuestions to consider• How did the two episodes of her disease differ with

regard to pathogenesis and to the kind of bacteria involved?

• Why did B. fragilis survive the first course of antibiotic treatment?

• Was she treated properly? What could have been done to lessen the likelihood of abscess formation?

• How does B. fragilis facilitate intra-abdominal abscess formation?

Gorbach’s experimentGorbach’s experiment

E. coli&

B. fragilisinjected

i.p.

RX GIVEN

None

Clindamicin

Gentamicin

Clindamycin & gentamicin

BacteroidesBacteroides spp. spp.• Obligate anaerobes• 25% of all colonic bacteria• Usually involved in infections resulting from

perforation of an abdominal viscus– ruptured appendix– diverticulitis– post-op after bowel surgery and/or dehiscence of a surgical

anastamosis

• Any Bacteroides spp. may be involved in a polymicrobial infection, but most abscesses contain B. fragilis

Survival features of Survival features of BacteroidesBacteroides• Bacterial enzymes digest complex polysaccharides

– Nutritional advantage– May improve human nutrition by digesting complex plant

polysaccharides in food (symbiosis with the host)– Can digest and consume human glycans, (e.g., mucin, hyaluronate,

chondroitin SO4)

– Neuraminidase: exposes sialylated polysaccharides to enzyme digestion (required for abscess formation)

• Bacteroides spp. are relatively aerotolerant– Human peritoneum and tissues are less anaerobic than the colon

What’s special about What’s special about B. fragilis B. fragilis ??

1. More aerotolerant than other species and more resistant to reactive oxygen species

• Possesses a superoxide dismutase (SOD)• Possesses catalase (CAT)

2H2O2 2H2O + O2

CAT

O2 + 2H2O 2H2O2

SOD.

What’s special about What’s special about B. fragilis B. fragilis ??

2. The outer membrane LPS (lipid A) is modified to be less toxic than that of E. coli• Allows for host tolerance of large numbers of

organisms without toxicity

What’s special about What’s special about B. fragilis B. fragilis ??

3. It has a complex capsular polysaccharide that is essential for abscess formation• Composed of at least 8 polysaccharides• Each is capable of transcriptional phase

variation (synthesis genes preceded by an invertable region containing a promotor)

• Polysaccharide A is essential for abscesses in animal models and is zwitterionic

Special features of Special features of B. fragilis B. fragilis CPCCPC

++++++++++

----------

Pol

ysac

char

ide

A

Activation of CD4+

T-lymphocytes

abscessformation

P

P

Gene transcribed

Gene silent

Phase variation of CPC synthesis genes

Antibiotic resistance in Antibiotic resistance in BacteroidesBacteroides

• Most carry a beta-lactamase gene (resistant to penicillin, ampicillin, 1st gen. ceph.)

• Harbors conjugative transposons– Can exchange genes with other Bacteroides and

with other species– ex. clindamycin resistance (only ~60% sensitive

now)

Abscess formationAbscess formation• Infectious inoculum is high• Spillage of intestinal contents into the

peritoneum most are killed by the immediate inflammatory response

• Containment by the omentum• Facultative bacteria establish first• Aerotolerant anaerobes survive• Microbial synergy is usually required

Microbial synergy in abscess formationMicrobial synergy in abscess formation

Bacteroidesspp.

E. coli and otherfacultative bacteria

Carbohydrate digestion

= more free sugars

Consumption of O2

+ decreased blood flow= more anaerobiasis

Response to bacteria in the peritoneumResponse to bacteria in the peritoneum

• Role of the omentum• Inflammatory mediators increase vascular

permeability plasma and fibrin influx– fibrous collagenous capsule forms around site– central area features acidic pH, live and dead

PMNs, and mixed bacterial flora– may include other Bacteroides, Clostridia, or

Peptostreptococcus spp.)

Treatment of peritonitis and peritoneal Treatment of peritonitis and peritoneal abscessesabscesses

• Abscesses must be drained surgically or by percutaneous catheters (+ repair any leak)

• Antibiotic therapy effective against colonic flora, including facultative and obligate anaerobic organisms– -lactams or cephalosporins + metronidazole– -lactam--lactamase inhibitor combinations– Carbepenems– Clindamycin is becoming less useful

Questions to considerQuestions to consider• How did the two episodes of her disease differ with

regard to pathogenesis and to the kind of bacteria involved?

• Why did B. fragilis survive the first course of antibiotic treatment?

• Was she treated properly? What could have been done to lessen the likelihood of abscess formation?

• How does B. fragilis facilitate intra-abdominal abscess formation?

Other Other BacteroidesBacteroides-associated diseases-associated diseasesand other obligate anaerobes of and other obligate anaerobes of

interestinterest

Case: Fever, cough, chest pain, and Case: Fever, cough, chest pain, and reallyreally bad breath bad breath

• A 53 year old man comes to the ED for fever and chest pain. He is coughing spasmodically with minimal sputum production.

• The patient is a heavy alcohol user and has had “blackouts” and seizures

• P.E. T=38.4. carious teeth noted, many fractured. Crackles over the left lung noted.

Case (continued)Case (continued)• He is admitted and started on ceftriaxone

for probably pneumonia• Later the same night, the patient starts

coughing copious amounts of grayish, putrid sputum that can be detected on the next ward.

• A chest xray is taken, and treatment with metronidazole is added.

Lung abscessLung abscess

Abhijit Datir, radiopaedia.org

Aspiration pneumoniaAspiration pneumonia

Source undetermined

Gram stain of mixed oral floraGram stain of mixed oral flora

Source undetermined

Oral, Gram-negative anaerobesOral, Gram-negative anaerobes• Common pathogens in dental infections, chronic

sinusitis, aspiration pneumonia, lung abscesses– Porphyromonas asaccharolytica, gingivalis, forsythus– Prevotella melaninogenicus (named for brown pigment

production)

• These species are usually (not always) sensitive to clindamycin. PCN+metronidazole usually works well.

• Infections are polymicrobial and usually include oral (viridans) streptococci, anaerobic strep, and other oral bacteria.

Case: pelvic inflammatory diseaseCase: pelvic inflammatory disease• A 24 year old woman presents with pelvic pain and

vaginal discharge.• She has been treated for gonorrhea in the past and

has had two prior episodes of the current illness in the past year.

• P.E. Temp=38C. There is lower abdominal tenderness in the RLQ and exquisite tenderness of the cervix and enlargement of the right Fallopian tube on pelvic exam

Case (continued)Case (continued)• A pregnancy test is negative• Because she has been unable to eat without

vomiting, she is admitted and treated with IV ceftriaxone, oral doxycycline, and oral metronidazole

• No cultures are obtained; an HIV test is negative• Why is metronidazole used?

PID microbiologyPID microbiology• Primary pathogens: gonococcus, chlamydia• Secondary pathogens:

– Facultative enteric organisms (e.g., E. coli)– GI and vaginal anaerobes

• Prevotella bivius and Prevotella disiens • Peptostreptococcus spp.

– Haemophilus

Case: neck and chest painCase: neck and chest pain• A 22 year old male who recently had an

prolonged episode of pharyngitis now presents with high fever, and exquisite pain, tenderness and swelling of his left neck for 2 days.

• This morning, he developed sharp pain in the left lower chest with deep breathing

• A blood culture is positive for an anaerobe

Lin D, Suwanantarat Nm Young RS. Hawaii Med J 2010; 69(7):161-3

Lin D, Suwanantarat Nm Young RS. Hawaii Med J 2010; 69(7):161-3

FusobacteriumFusobacterium

Source undetermined

Lemierre’s syndromeLemierre’s syndrome• Or “post-anginal sepsis” (very rare)• Occurs after prolonged or severe

pharyngitis• Septic thrombophlebitis with

Fusobacterium necrophorum (probably from the mouth) associated with septic pulmonary emboli to the lungs

What have you learned about Gram-What have you learned about Gram-negative, obligate anaerobesnegative, obligate anaerobes

• Bacteroides participate in intra-abdominal abscesses when intestinal contents spill into the peritoneum

• Formation of abscesses is a synergistic process involving anaerobes & facultative bacteria

• B. fragilis has special capacity to tolerate oxygen and to induce abscess formation via its CPC

• Other Bacteroides-like anaerobes are involved in polymicrobial dental, lung, or pelvic infections

Additional Source Informationfor more information see: http://open.umich.edu/wiki/CitationPolicy

Slide 10: Yu J, Fulcher AS, Turner MA, Halvorsen RA. Helical CT Evaluation of Acute Right Lower Quadrant Pain: Part I, Common Mimics of Appendicitis Am J Roentgenol. 2005;184(4):1136-1142. Resource: medscape.com

Slide 12: Source undetermined

Slide 13: CDC: Public Health Image Library/Dr. V.R. Dowell, Jr.,1972, http://phil.cdc.gov/phil_images/20030203/6/PHIL_3084_lores.jpg

Slide 31: Abhijit Datir, Lung Abscess, Radiopaeidia.org, http://radiopaedia.org/articles/lung_abscess

Slide 32: Source undetermined

Slide 33: Source undetermined

Slide 39 & 40: Lin D, Suwantarat N, Young RS. Lemierre’s Syndrome mimicking leptospirosis. Hawaii Med J. 2010; 69(7):161-63. http://www.hawaiimedicaljournal.org/69.07.161.htm

Slide 41: Source undetermined